i think it will be slower cos there is a need for interfacing. i had a problem where i have almost 100 sub-domains and each subdomains have element scatteredd (i.e. disjointed) it cannot even be imported into CFX from ICEM.

I assume you meant running your case in parallel. If you run a calculation using more than one processor, first thing you need to check is what would be your mesh size. If your mesh size is large, it would be faster using more machine working together. It depends on percentage of mesh 'overlap' between each partition. If computing overhead is not due to large mesh size (for example, due to radiation calculation), running in parallel is not going to be helpful, may slow down, as jemteo pointed.